Tuesday, March 31, 2015

You know how it is: you hear something enough, you
believe it's true. Look on the side of any box of water soluble
fertilizer, or any organic gardening guide, and there will be
instructions on foliar feeding: spraying a water soluble fertilizer onto
the leaves of a plant, as an alternative source of nutrition for the
plant.

Awhile back on the radio show, a tempest in a teapot developed when Milo Shammas, the President of the Dr. Earth line of organic products, mentioned that the best way to apply a foliar fertilizer, which he endorses, is in as fine a spray as possible. In his corner, Rodale's Encyclopedia of Organic Gardening,
which states: "Plants can absorb liquid fertilizers through both their
roots and through their leaf pores. Foliar feeding can supply nutrients
when they are lacking or unavailable in the soil, or when roots are
stressed. It is especially effective for giving fast growing plants like
vegetables an extra boost during the growing season...Any sprayer or
mister will work, from hand trigger units to knapsack sprayers. Set your
sprayer to emit as fine a spray as possible."

Disagreeing with the "fine spray" approach is another organic advocate
and frequent guest of the radio shows, Steve Zien of the Sacramento
area-based organic consulting service, Living Resources Company.
Zien says, "Some time ago I read a few studies that indicated that most
of that (spraying with a fine spray) was not necessary. The studies
used radio isotopes to follow the nutrients in the foliar fertilizer.
They found that it got into the plant even when the water droplets were
large. Another study indicated that even with the best spray equipment
making the smallest water droplets possible with today's technology, the
water droplets were still too large to physically enter the plant. They
concluded that water droplet size is not important when foliar feeding.
Other studies have shown that foliar fertilizer can even be absorbed by
branches and tree trunks. These two facts indicate that where you
spray is also not critical. Numerous studies have shown that foliar
feeding is much more efficient at getting the nutrients absorbed and to
the entire plant and more rapidly as well.
I think all the studies emphasize that even with all the benefits of
foliar feeding, it cannot be considered a substitute for proper soil
nutrition, and I fully agree with that. You need to feed the soil
foodweb for healthy, pest resistant plants.I no longer worry about where
I apply the foliar fertilizer. I try to apply it to as many plant
surfaces as possible but do not worry about paying attention to the
undersides of the leaves."

Throwing cold water on both those practices are a couple of college
educators, Deborah Flower of the Horticulture Department of American
River College in Sacramento; and, Linda Chalker-Scott of the Horticulture Department at Washington State University and author of the award-winning book, "The Informed Gardener", who says this about foliar feeding:
"The existing research does not justify foliar fertilization of
landscape plants as a general method of mineral nutrition. It can be
useful for diagnosing deficiencies; for instance, spraying leaves with
iron chelate can help determine if interveinal chlorosis is from iron
deficiency. It would obviously have a benefit for those landowners with
landscape fruit trees that perpetually have flower or fruit disorders
associated with micronutrient deficiencies. Applying fertilizers to
leaves (or the soil) without regard to actual mineral needs wastes time
and money, can injure plant roots and soil organisms, and contributes to
the increasing problem of environmental pollution. The bottom line:
• Tree and shrub species differ dramatically in their ability to absorb foliar fertilizers.
• Proper plant selection relative to soil type is crucial to appropriate mineral nutrition.
• Foliar spraying is best accomplished on overcast, cool days to reduce leaf burn.
• In landscape plants, foliar spraying can test for nutrient deficiencies, but not solve them.
• Micronutrients are the only minerals that are effectively applied through foliar application.
• Foliar application will not alleviate mineral deficiencies in roots or subsequent crown growth.
• Foliar spraying is only a temporary solution to the larger problem of soil nutrient availability.
• Minerals (especially micronutrients) applied in amounts that exceed a
plant’s needs can injure or kill the plant and contribute to
environmental pollution.
• Any benefit from foliar spraying of landscape trees and shrubs is minor considering the cost and labor required."

Chiming in is Deborah Flower of American River College: "I have been
reading 'Plant Physiology' by Taiz and Zeiger, and 'Mineral Nutrition of Higher Plants' by Horst Marschner. The
latter discusses foliar feeding in chapter 4. It says: there are small
pores in the cuticle through which minerals can enter the plant. These
pores are in highest density near guard cells around stomata and at base
of trichomes (hairs, scales, etc.). They are tiny and lined with
negative charges. So, only very small (less than one nanometer in
diameter) cations and uncharged molecules will enter these openings.
It says leaves do absorb ions, but (pages 123-125):
Rate of uptake is VERY low.
Rate of uptake varies between species and growing conditions. Plants
with thicker cuticle (due to species or growing conditions) absorb less.
Older leaves have lowest rate of uptake due to leaky plant cells that
fill intercellular spaces, which is where ions travel in the leaf.
A very high concentration of ions is needed outside the leaf to get any into the leaf.
The supply of nutrients in the leaf from foliar feeding is temporary.
There is limited movement of nutrients from leaves to other plant parts.
Urea can enter leaves through these openings (ammonia and nitrate
cannot), because it is an uncharged particle, but can cause damage in
the leaf, due to nutrient imbalance in the leaf once it is absorbed.
Surfactants should be used with all foliar feeding to increase surface spread of spray.

So, my opinion is that yes, plants do absorb nutrients through
their leaves (neither book mentioned absorbtion through branches or
trunks) but the amount is very small, nutrients do not travel far from
point of entry, and there is lots of nutrient run-off during the
process, which can lead to pollution. Therefore, foliar feeding is not
effective as the primary source of nutrients for plants. I disagree that
foliar feeding gets nutrients to all parts of the plant. There is lots
of evidence that fertilizer that gets into the leaf migrates little to
other parts of the plant. It stays in the leaf or travels to a strong
sink like a fruit. Foliar feeding can correct micro-nutrient
deficiencies in leaves and some fruit, but until the nutrition is
balanced in the root zone, the symptom will continue to appear in new
plant parts. Many of my students seem to believe foliar feeding is
better for the plant than nutrient absorption by roots, and that
concerns me. Foliar feeding can be used to correct some nutritional
problems, primarily in production situations, but should not be relied
on as the primary source of nutrients for the plant. If people are
foliar feeding I believe most of the nutrients being absorbed by the
plant are entering the roots, probably after running off the plant onto
the soil."

Milo Shammas, of Dr. Earth, responds: "Fred, all very true and I agree
with her, I do not recommend foliar feeding as the primary source of
nutrients. Whatever runs off the foliage will ultimately be absorbed by
the root system. Nothing can replace ion absorption through the root
system.
Foliar feeding as a supplement? Yes
Is it effective? Yes
Would I depend on it solely? No
Is there harm in using it? No
Do younger leaves absorb it better? Yes
I own and manage 45 acres of organic walnuts and I personally spray my
ranch with Dr. Earth liquid solution twice a year, I do spend the money
on it, I have conducted the efficacy and I know it works, I do believe
in it, I do endorse it, I do not depend on it."

After standing back, listening to all this, I have come to the
conclusion: although foliar feeding may have minimal value, it does have
a bigger, positive effect: washing off bad bugs from the leaves. Of
course, a spray of water can accomplish the same thing.

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Fred Hoffman is the producer/host of the "KFBK Garden Show" on KFBK-Sacramento (1530-AM and 92.5-FM) each Sunday morning from
8:30 a.m. to 10 a.m., followed by "Get Growing" on Talk 650 KSTE, 10 a.m. to Noon.
Outside Northern California? Listen to the shows, live, via the KFBK.com and KSTE.com websites or as podcasts from those websites.